<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
<input type="file" id="file">
<script>
    document.getElementById('file').addEventListener('change',function (e) {
        let fileObj = e.target.files[0]
        let that = this;
        compressFile(fileObj,function (files) {
            console.log(files)

            that.value = '' // 加不加都行，解决无法上传重复图片的问题。

        })

    })

    /**
     * 压缩图片
     * @param file input获取到的文件
     * @param callback 回调函数，压缩完要做的事，例如ajax请求等。
     */
    function compressFile(file,callback) {
        let fileObj = file;
        let reader = new FileReader()
        reader.readAsDataURL(fileObj) //转base64
        reader.onload = function(e) {
             let image = new Image() //新建一个img标签（还没嵌入DOM节点)
            image.src = e.target.result
            image.onload = function () {
                let canvas = document.createElement('canvas'), // 新建canvas
                    context = canvas.getContext('2d'),
                    imageWidth = image.width / 2,    //压缩后图片的大小
                    imageHeight = image.height / 2,
                    data = ''
                canvas.width = imageWidth
                canvas.height = imageHeight
                context.drawImage(image, 0, 0, imageWidth, imageHeight)
                data = canvas.toDataURL('image/jpeg') // 输出压缩后的base64
                let arr = data.split(','), mime = arr[0].match(/:(.*?);/)[1], // 转成blob
                    bstr = atob(arr[1]), n = bstr.length, u8arr = new Uint8Array(n);
                while (n--) {
                    u8arr[n] = bstr.charCodeAt(n);
                }
                let files = new window.File([new Blob([u8arr], {type: mime})], 'test.jpeg', {type: 'image/jpeg'}) // 转成file
                callback(files) // 回调
            }
        }
    }
</script>
</body>
</html>